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Thursday, February 23, 2012

Changed Block Tracking (CBT) on virtual machines

Changed Block Tracking (CBT) is a VMware feature that helps perform incremental backups. VMware Data Recovery uses this technology, and so can developers of backup and recovery software.

Virtual machines running on ESX/ESXi hosts can track disk sectors that have changed. This feature is called Changed Block Tracking (CBT). On many file systems, CBT identifies the disk sectors altered between two change set IDs. On VMFS partitions, CBT can also identify all the disk sectors that are in use.

Virtual disk block changes are tracked from outside virtual machines, in the virtualization layer. When software performs a backup, it can request transmission of only the blocks that changed since the last backup, or the blocks in use.

The CBT feature can be accessed by third-party applications as part of the vSphere APIs for Data Protection (VADP). Applications call VADP to request that the VMkernel return blocks of data that have changed on a virtual disk since the last backup snapshot.

For CBT to identify altered disk sectors since the last change ID, the following items are required:

The host must be ESX/ESXi 4.0 or later.

The virtual machine owning the disks to be tracked must be hardware version 7 or later.

I/O operations must go through the ESX/ESXi storage stack. So NFS is supported, as is RDM in virtual compatibility mode, but not RDM in physical compatibility mode. Of course VMFS is supported, whether backed by SAN, iSCSI, or local disk.

CBT must be enabled for the virtual machine (see below).

Obviously, virtual machine storage must not be (persistent or non-persistent) independent disk, meaning unaffected by snapshots.

For CBT to identify disk sectors in use with the special "*" change ID, the following items are required:

The virtual disk must be located on a VMFS volume, backed by SAN, iSCSI, or local disk. RDM is not VMFS.

The virtual machine must have zero (0) snapshots when CBT is enabled, for a clean start.

In some cases, such as a power failure or hard shutdown while virtual machines are powered on, CBT might reset and lose track of incremental changes. Likewise offline Storage vMotion, but not online Storage vMotion, could reset but not disable CBT.